1. Field of the Invention
The present invention relates to an emitting compound and an organic light emitting diode (OLED) device and more particularly to an emitting compound having excellent power efficiency and hole injecting and transporting properties and an OLED device using the same.
2. Discussion of the Related Art
Recently, requirement for flat panel display devices, such as a liquid crystal display device and a plasma display panel, is increased. However, these flat panel display devices have relatively slow response time and narrow viewing angle in comparison to the cathode ray tube (CRT).
An organic light emitting diode (OLED) device is one of next-generation flat panel display devices being capable of resolving the above problems and occupying small area.
The related art inorganic type OLED device requires a driving voltage higher than 220V, and it is difficult to be a large size because the inorganic type OLED device should be fabricated in a vacuum condition. Particularly, it is very difficult to provide high efficiency blue color images.
To overcome these problems, an organic type OLED device using organic materials is developed. The OLED device is self-emitting type display device. The OLED device emits light by injecting electrons from a cathode as an electron injection electrode and holes from an anode as a hole injection electrode into an emission compound layer, combining the electrons with the holes, generating an exciton, and transiting the exciton from an excited state to a ground state. Elements of the OLED device can be formed on a flexible substrate such as a plastic substrate. In addition, the OLED device has advantages in the viewing angle, the driving voltage, the power consumption and the color purity. Moreover, the OLED device is adequate to produce full-color images.
The emitting principle may be classified into fluorescent emission and phosphorescent emission. In the fluorescent emission, the organic molecule in the singlet exited state is transited to the ground state such that light is emitted. On the other hand, in the phosphorescent emission, the organic molecule in the triplet exited state is transited to the ground state such that light is emitted.
The atoms in the organic material in the OLED device share electron to form covalent bonds. In this process, the atomic orbital is transited into the molecular orbital. In the molecular orbital, the bonding molecular orbital and the antibonding molecular orbital are generated by a pair of orbitals. In this instance, the band formed by the bonding molecular orbitals is referred to as a valence band, and the band formed by the antibonding molecular orbitals is referred to as a conduction band. The valence band having a highest energy level is referred to as a highest occupied molecular orbital (HOMO), and the conduction band having a lowest energy level is referred to as a lowest unoccupied molecular orbital (LUMO). In addition, a difference between the HOMO and the LUMO is referred to as a band-gap.
When the emitting material layer emits light corresponding to an band-gap energy, the singlet exciton having 0 spin and the triplet exciton having 1 spin are generated with a ratio of 1:3. The ground state of the organic material is the singlet state such that the singlet exciton can be transited to the ground state with emitting light. However, since the triplet exciton can not be transited with emitting light, the internal quantum efficiency of the OLED device using the fluorescent material is limited within 25%.
On the other hand, if the spin-orbital coupling momentum is high, the singlet state and the triplet state are mixed such that an inter-system crossing is generated between the singlet state and the triplet state and the triplet exciton also can be transited to the ground state with emitting light. The phosphorescent material can use the triplet exciton as well as the singlet exciton such that the OLED device using the phosphorescent material may have 100% internal quantum efficiency.
Accordingly, the development is focused on the phosphorescent material having higher efficiency than the fluorescent material. Recently, iridium complex, e.g., (bis(2-(2′-benzothienyl)-pyridinato-N,C3′)iridium(acetylacetonate)) (Ir(btp)2(acac)) as a red phosphorescent material, bis(2-phenylpyridine) iridium(III) acetylacetonate ((ppy)2Ir(acac)) as a green luminescent material, and Iridium (III) bis[2-2′,4′-difluorophenylpyridinato-N,C2′]picolinate (FIrpic) as a blue phosphorescent material, is introduced. In addition, a blue phosphorescent material, e.g., material, 4,4,N,N-dicarbazole-biphenyl (CBP) and N,N-dicarbazole-3,5-benzene (mCP), as a host is introduced.
However, the blue emitting material has problems in the color purity, the luminescent efficiency and the lifetime resulting from bad thermal stability. As a result, it is not adequate to the OLED for producing high quality blue images, and new blue emitting material is required.